download the nixos-minimal-16.03.714.69420c5-x86_64-linux.iso and use unetbootin to deploy it to an USB stick. afterwards mount the first partition of the USB-stick and append this to the syslinux.cfg file's kernel command line:

console=ttyS0,115200n8

info: using the serial console you can see the GRUB output, see the kernel's output after boot and finally get a shell.

booting from the USB stick

the apu 2c4 features corebios and the process is straight forward, just hit F10 and select the USB stick